1. Field of the Invention
The present invention relates to a process for epitaxially growing II-VI compound semiconductors containing sulfur as a composition element, such as zinc sulfide (ZnS) and zinc sulfo-selenide (ZnSSe).
2. Description of the Prior Art
II-VI compound semiconductors such as ZnS and ZnSSe have a wide forbidden band gap of the direct transition type and are highly promising as materials for devices for emitting high-luminance light in the region of ultraviolet to visible short wavelengths. However, control of the conduction type is indispensable to realize high-luminance luminescent devices. Research is under way on the growth of high-quality crystals with use of the molecular beam epitaxial (MBE) process, metal-organic vapor phase epitaxy (MOVPE) and like low-temperature epitaxial techniques and on the control of the conduction type.
In the MBE process for preparing II-VI compound semiconductors, such as ZnS and ZnSSe, containing sulfur, the sulfur molecular beam to be used is produced conventionally by decomposing hydrogen sulfide by heating ("SINGLE CRYSTAL GROWTH OF ZnS BY THE METHOD OF GAS SOURCE MBE," Journal of Crystal Growth, 76(1986) 440-448), or by heating a sulfide such as ZnS or simple sulfur using a Knudsen cell (K-cell)("THE PREPARATION OF CONDUCTIVE ZnS FILMS BY USING MBE WITH A SINGLE EFFUSION SOURCE," Journal of Crystal Growth, 67(1984) 125-134; "MOLECULAR BEAM EPITAXIAL GROWTH AND STRUCTURE CHARACTERIZATION OF ZnS ON (001)GaAs," ibid., 86(1988) 303-310; "Growth of ZnS Bulk Single Crystals and Homoepitaxial Growth of ZnS by Molecular Beam Epitaxy," Extended Abstract of the 19th Conference on Solid State Device and Materials, Tokyo, 1987, pp. 247-250).
However, when hydrogen sulfide or other sulfide is to be used, it is difficult to obtain the material with a high purity, and impurities become inevitably incorporated into the epitaxial film. Furthermore, the use of hydrogen sulfide involves the problem that molecules not participating in the growth , such as those of undecomposed hydrogen sulfide and hydrogen gas resulting from thermal decomposition, are present conjointly with the sulfur molecular beam on the grown surface. When ZnS or like sulfide is used, a zinc molecular beam occurs at the same time, and it is impossible to control the supplies of sulfur and zinc independently of each other, hence another problem.
On the other hand, the method of producing a molecular beam by heating solid sulfur with a K-cell has advantages. The material is readily available with a high purity, no vapor occurs other than the vapor of sulfur, and the supply of sulfur to the grown surface is controllable independently. Nevertheless, the method has the following problem.
The evaporation temperature required for giving a sulfur molecular beam with a suitable intensity of about 1.times.10.sup.-7 to about 1.times.10.sup.-5 torr is usually as low as about 100.degree. C., so that the sulfur molecules projected on the surface of the layer to be grown have low thermal energy. Consequently, the sulfur molecules fail to diffuse fully over the surface to result in three-dimensional growth, making it impossible to inhibit occurrence of defects and presenting difficulty in forming a flat grown surface.
Thus, the conventional MBE process encounters difficulties in forming a ZnS or ZnSSe single crystal epitaxial film of high quality suitable as the light-emitting layer of highly efficient luminescent devices.
The present invention has been accomplished to overcome the above problems.
Incidentally, it has been proposed for the preparation of GaAs semiconductors to heat the vapor for supplying a molecular beam after the vapor has been produced ("A correlation between electron traps and growth processes in n-GaAs prepared by molecular beam epitaxy," Appl. Phys. Lett. 36, No. 4, 15 February 1980; "The effect of As.sub.2 and As.sub.4 molecular beam species on photoluminescence of molecular beam epitaxially grown GaAs," ibid., 37, No. 4, 1 August 1980), whereas this method has not been applied to MBE for sulfur-containing II-VI compound semiconductors.